There are many examples wherein there is a requirement for polymeric fastener, particularly in underground mining, chemical plant installation and marine work. The requirement for polymerics in underground mining is to avoid the tedious withdrawal of rock bolts, which, if polymeric, can be cut away by a mining machine along with the rock. For chemical, aerospace and marine applications, polymeric fasteners are required for anticorrosion properties, stainless steel quite often being only partially valuable, in certain circumstances corroding and in some circumstances being subject to stress corrosion which makes stainless steel an unreliable material in these applications. Titanium is an alternative metal which can be used in many instances because of its excellent anticorrosive and strength properties but titanium is a very expensive metal. Another requirement is light weight, particularly for aerospace applications, steel rock bolts for example often being too heavy for easy handling.
Many attempts have been made to utilize polymeric fasteners, the most usual construction being a bolt having a plurality of longitudinally extending reinforcing fibers, the thread of the bolt being out with a diamond cutting tool which can form the threads without excessive wear, but in cutting through some of the fibers the diamond cutting tool removes the continuity of the tensile reinforcements at the locality of the threads, that is, where they are most needed. It has already been established that under average fastener conditions wherein linear elastic metals having isotropic properties are used, e.g. mild steel, and the male and female threads have identical pitches, upon a tensile stress being applied to the nut, between 30% and 60% of the stress is taken by the first thread. Various provisions are sometimes made to spread the load, for example by having the pitch of the female threads slightly greater than the pitch of the male thread, but even with the efforts which have been made there is still an excessive compression of the first female thread and stretching of the first male thread, and thread failure is likely to occur in prior art polymeric bolts in domino fashion. It should be noted that a continuous unidirectional fiber reinforced plastic has a generally straight line stress strain relationship to failure, and does not have the advantage of most metals of deforming slightly when it reaches a yield point. Thus for example, the best available prior art plastic nut/rod combination known to the applicants has an ultimate strength of only 100 kN. Ordinary mild steel rod of the same shank diameter has a yield point of about 95 kN and an ultimate tensile strength of about 140 kN.